Then the Cold War turned hot: Vladimir V. Putin ordered Russian forces to invade Ukraine. The repercussions were immediate, and far-reaching.
Now, following the launch of Russia’s full-scale invasion on Feb. 24, the largest mobilization of forces Europe has seen since 1945 is underway. So far, Moscow has been denied the swift victory it anticipated, and has failed to capture major cities across the country, including Kyiv, the capital. It has been weighed down by an ill-prepared military and has faced tenacious resistance from Ukrainian soldiers and civilian resistance fighters. Still, Russia has superior military might, and Mr. Putin has indicated that his ultimate goal is to capture Kyiv, topple Ukraine’s democratically elected government, and subsume the country into Russia’s orbit.
The invasion threatens to destabilize the already volatile post-Soviet region, with serious consequences for the security structure that has governed Europe since the 1990s. Mr. Putin has long lamented the loss of Ukraine and other republics when the Soviet Union broke apart. Now, diminishing NATO, the military alliance that helped keep the Soviets in check, appears to be part of his mission. Before invading, Russia made a list of far-reaching demands to reshape that structure — positions NATO and the United States rejected.
With the war grinding on, U.S. intelligence agencies say Mr. Putin has been frustrated by the slow pace of the military advance and Russian commanders have been increasingly intensifying indiscriminate attacks on civilian targets and infrastructure and resorting to tactics used in previous wars in Chechnya and Syria. Mariupol. Kharkiv. Chernihiv. Sumy. Okhtyrka. Hostomel. Irpin. The list of Ukrainian cities turned to ruins keeps growing.
The war has unleashed a devastating humanitarian toll and claimed thousands of lives. It has also prompted more than three million people to flee Ukraine, spurring what the United Nations has called the fastest-growing refugee crisis since World War II.
In the besieged southern port city of Mariupol, which has become a potent emblem of the human toll of war, a Russian strike on March 16 destroyed a theater where hundreds of people had been sheltering, including children. The city has no electricity or water, and people have been digging trenches to accommodate the mounting numbers of bodies.
Several rounds of diplomatic talks between Russia and Ukraine have failed to stop the war. The United States and the European Union have mobilized to impose some of the toughest economic sanctions ever on Mr. Putin’s government. Hundreds of Western businesses — manufacturers, oil companies, retailers and fast-food chains like McDonald’s — have suspended operations in Russia, turning back the clock on the country’s opening to the west.
After the Soviet Union collapsed in the early 1990s, NATO expanded eastward, eventually taking in most of the European nations that had been in the Communist sphere. The Baltic republics of Lithuania, Latvia and Estonia, once parts of the Soviet Union, joined NATO, as did Poland, Romania and others.
As a result, NATO moved hundreds of miles closer to Moscow, directly bordering Russia. And in 2008, it stated that it planned — some day — to enroll Ukraine, though that is still seen as a far-off prospect.
Mr. Putin has described the Soviet disintegration as one of the greatest catastrophes of the 20th century that robbed Russia of its rightful place among the world’s great powers. He has spent his 22 years in power rebuilding Russia’s military and reasserting its geopolitical clout.
The Russian president calls NATO’s expansion menacing, and the prospect of Ukraine joining it a major threat. As Russia has grown more assertive and stronger militarily, his complaints about NATO have grown more strident. He has repeatedly invoked the specter of American ballistic missiles and combat forces in Ukraine, though U.S., Ukrainian and NATO officials insist there are none.
Mr. Putin has also insisted that Ukraine is fundamentally parts of Russia, culturally and historically.
East-West relations worsened drastically in early 2014, when mass protests in Ukraine forced out a president closely allied with Mr. Putin. Russia swiftly invaded and annexed Crimea, part of Ukraine. Moscow also fomented a separatist rebellion that took control of part of the Donbas region of Ukraine, in a war that still grinds on, having killed more than 13,000 people.